1. Field of the Invention
This invention relates to articles of manufacture including a polyamide barrier layer.
2. Description of the Prior Art
There has been an increasing demand for suitable plastic materials to replace glass and metals in the packaging industry, particularly involving food, beverage and pharmaceutical products. However, for such applications, the plastic materials must have, in addition to good processability and mechanical properties, sufficiently high barrier resistant to oxygen, moisture and organic substances (flavor, odor and ingredients), and in the case of carbonated beverages, to carbon dioxide. There are many classes of polymers with specific molecular structures that can achieve good gas barrier resistance to oxygen and carbon dioxide. However, very often materials with good oxygen barrier properties are poor moisture barriers and vice versa, e.g., polar polymers containing hydroxylic groups such as cellophane and polyvinyl alcohol are excellent oxygen barriers but are extremely hygroscopic, and lose their barrier property to a significant extent when wet. Hydrophobic polymers such as polyolefins are good moisture barriers but poor oxygen barriers. Polymers containing a balance of hydrophobic and hydrophilic structures such as ethylene-vinyl alcohol copolymers and nylons (6 and 6,6) have good to excellent barrier properties at low humidity levels but at high humidities become more permeable to oxygen due to the plasticizing action of small amounts of water absorbed. For example, the permeability of nylon 6 to oxygen increases from about 2. cc. mil/100 in..sup.2 24 hrs. atm. at 0% RH to 15 cc. mil/100 in..sup.2 24 hrs. atm. at 100% RH. Similarly ethylene-vinyl alcohol shows more than 100 fold increase in permeability to oxygen with humidity, increasing from 0.01-0.06 cc. mil/100 in..sup.2 24 hrs. atm at 0% RH to 1-2 cc. mil/100 in..sup.2 24 hrs. atm at 100% RH. While both nylon 6 and ethylene-vinyl alcohol are currently being used in the packaging industry, usually as part of a multi-ply package, due to favorable economics and balance of barrier properties, there is still a need for an article of manufacture including a film barrier layer which would exhibit both excellent oxygen and moisture barrier properties or a moisture-insensitive oxygen barrier characteristic.